1. Field of the Invention
The present invention generally relates to reducing power consumed by disk drives. More particularly, the present invention relates to disk drives employing power source modulation for reducing power consumption, such as when the disk drive is idling.
2. Description of the Prior Art
In hard disk drives, data is stored on magnetic media disks in concentric data tracks, which are divided into groups of data sectors. Disks are typically stacked on a spindle assembly. The spindle assembly is mechanically coupled to a spindle motor which rotates the disks at a substantially constant operating spin-rate. A spindle motor driver typically includes power metal oxide semiconductor field effect transistors (MOSFETs) to drive the spindle motor. A microprocessor is typically employed to ascertain when to apply a run signal, a coast signal, or a brake signal to the spindle motor driver to control the operation of the spindle motor.
The torque developed (T.sub.d) by the spindle motor is given by the following Equation I: EQU T.sub.d =Kt*I.sub.m Equation I
Where:
Kt is the torque constant of spindle motor; PA1 I.sub.m is spindle motor current; PA1 Power dissipation in the windings of the spindle motor are given by the following Equation II: EQU P=I.sub.m.sup.2 *R.sub.m Equation II PA1 where: PA1 where:
R.sub.m is the total resistance of the spindle motor and wire connections between the spindle motor and the power MOSFETs. PA2 R.sub.fet is the resistance in the power MOSFETs that are turned on for controlling current Im flowing through the spindle motor.
Power dissipation in the power MOSFETs are given by the following Equation III: EQU P=I.sub.m.sup.2 *R.sub.fet Equation III
It is known to increase the RPM of the spindle motor to reduce rotational latency and increase disk transfer rate in the disk drive. However, as disk drives employ spindle motors operating at higher RPMs (such as 10,000 RPMs and higher), it is necessary to offset increased drag torque opposing the developed torque T.sub.d in order to maintain the spindle motor at a substantially constant operating spin-rate. As can be shown by Equations I-III above, an increased developed torque T.sub.d required to off-set the increased drag torque to maintain the higher operating spin-rate of the spindle motor can be obtained by increasing the spindle motor current I.sub.m. Unfortunately, increasing the spindle motor current I.sub.m causes a corresponding heating of the windings of the spindle motor as shown by the above power dissipation Equation II and also a corresponding heating of the power MOSFETs used to drive the spindle motor as shown by the above Equation M. The additional heat generated by the disk drive can cause heat related breakdown of components in the disk drive, including the spindle motor windings and the power MOSFETs themselves.
In addition, power supplies in high performance computer systems which employ disk drives having spindle motors operating at high RPMs typically limit the average current output from the computer system's power supply circuitry to reduce the cost of the computer system. An increased spindle motor current I.sub.m necessary to maintain the higher operating spin-rate may cause an average spindle motor current to exceed the average current output from the computer system's power supply.
It is known to employ a power saving operation to reduce power consumption when the disk drive is idling during an idle period (i.e., no read or write operations are being performed). A power saving operation may include parking the head and spinning-down the disk drive to a stop during the idle period. U.S. Pat. No. 5,412,519 (the "Buettner patent") discloses a power saving operation that spins down the spindle motor to the minimum rotational velocity that is required for the head to continue flying above the disk during the idle period. A disadvantage with such power saving operations is that there is a significant recovery time to resume read and write operations after the idle period.
For reasons stated above, there is a need to reduce power consumption in the spindle motor and its power MOSFETs, especially in high performance disk drives operating at 10,000 RPMs and higher. In addition, there is a need to reduce the power consumption in the spindle motor and its power MOSFETs without significantly increasing the recovery time to resume read and write operations after an idle period.